1. Field of the Invention
The present invention relates to a method of operating an image obtained by a color image pickup apparatus, such as a film camera, a digital camera and a video camera, into a proper image, and a program and apparatus used for the method. The present invention relates more particularly to image processing that makes inconspicuous an originally nonexistent blur color distribution around a bright image.
2. Description of the Related Art
A color image pickup system generates originally nonexistent artifacts in blue, in red, and in purple as a blend of blue and red in a blur shape, around a bright part on an image due to a chromatic aberration of an imaging optical system. This blur is referred to as a color blur or a purple fringe.
A combination of plural lenses having different dispersions can optically reduce the chromatic aberration to some extent. However, as a small size of a digital camera progresses, high resolution image sensors (or image pickup devices) and smaller optical systems are increasingly demanded, and it is difficult to sufficiently reduce the chromatic aberration only by the optics. Therefore, the image processing is required to reduce an artifact.
The chromatic aberration is roughly classified into a lateral chromatic aberration (chromatic aberration of magnification) and a longitudinal (or axial) chromatic aberration. When the light emitted from a light source 10 images on a focal plane 12 via an imaging optical system 11, as shown in FIG. 1, the lateral chromatic aberration appears as shifts of imaging positions of a B ray 13 having a wavelength of about 450 nm, a G ray 14 having a wavelength of about 550 nm, and a R ray 15 having a wavelength of about 650 nm in the image surface direction. On the other hand, when the light emitted from a light source 20 images on a focal plane 22 via an imaging optical system 21, as shown in FIG. 2, the longitudinal chromatic aberration appears as shifts of imaging positions of the RBG rays 23-25 in the optical-axis direction.
As disclosed in U.S. Pat. No. 6,724,702, the lateral chromatic conversion of a digital imaging system of a primary color system can be corrected through a geometric conversion that applies a different deformation to each color plane of RGB.
On the other hand, the longitudinal chromatic aberration means, in an image that is focused with the G plane as a central wavelength of the visible light region, for example, that the image is out of focus with respect to the R and B planes that are ends of the visible light. This cannot be corrected by the geometric conversion although it is effective to the lateral chromatic aberration. The longitudinal chromatic aberration can be made inconspicuous by lowering the chroma of a color blur area as disclosed in Japanese Patent Laid-Open No. (“JP”) 2001-145117, or can be corrected by applying a different contour emphasis process to each color plane of RGB as disclosed in JP 2003-018407.
In addition, a color blur reduction process is available even in the motion picture imaging, as disclosed in JP 2006-135744, for a color blur generating portion by searching for it using the previous motion picture image frame, and by specifying a target region in this motion picture image frame to some extent.
However, a chroma lowering process disclosed in JP 2001-145117 is a process that degrades a color of a high brightness pixel down to a brightness level of another color. This process is effective in reducing unnaturalness by erasing a color of the color blur, but the original colors of a subject can sometimes be also influenced and the image is likely grayish irrespective of the existence of the color blur.
The process disclosed in JP 2003-018407 utilizes a deconvolution and its approximate contour emphasis process, but has a problem of a difficulty in obtaining a precise aberration function necessary to obtain a proper result. It is difficult to precisely recognize an aberration function corresponding to each of huge imaging conditions that include a zoom position, a stop value, a focus position, a size of each lens, and an assembly state. In addition, the deconvolution can be used only in the linear reaction area of an image sensor, and cannot reduce a color blur around the saturation pixel.
When each of these prior art chromatic aberration reduction processes is applied to a motion picture image obtained by taking a moving high brightness subject, discontinuous processes of a time series of an application and a non-application of the chromatic aberration reduction process generate unnatural motion picture images, bringing a sense of discomfort to a viewer.
The color blur reduction process disclosed in JP 2006-135744 utilizes the previous motion picture image frame and reduces a calculation load, but does not have an effect of mitigating a sense of discomfort caused by the discontinuous color blur reduction processes.